A method of the above mentioned general type is known from the publication ATMEL “Wireless Design Guide/Data Book 2001”, page 233, published by ATMEL Germany GmbH of Heilbronn, Germany. In this reference, an output voltage, which is proportional to the input signal of a receiving antenna, is generated by means of a so-called “RSSI (Received Signal Strength Indicator) amplifier”.
The output voltage thus represents a measure for the received field strength of the input signal. In general, this is achieved by cascading individual amplifier stages, of which ever more stages are driven into saturation as the input signal level rises. A signal proportional to the field strength of the received signal can be derived from the cumulative value of the output currents of the amplifier stages. The disadvantage of using RSSI amplifiers is the high current consumption of the method. Moreover, such amplifiers have long turn on and turn off transient times to achieve stable conditions, especially with quickly changing input signal levels which, particularly in the case of pulse-pause modulated signals, make it necessary to adapt the length of the field gap to the maximum attainable amplitude height in the input signal in order to ensure reliable data transmission. This substantially reduces the data transmission or transfer rate.
Among other uses, methods for gaining field strength information are used in systems for contactless data transmission. An important field of application is for identification systems which are, for example, used in an automotive vehicle for controlling access to the vehicle. Such identification systems consist of a base unit and one or more transponders. In this case, the distance between the transmitter and the receiver can be determined by means of the field strength information. Thus, this information makes it possible to increase the security against an unauthorized passing-on or relaying of the data transmission.